This invention concerns the treatment of gaseous effluent containing polluting matter which is susceptible to catalytic oxidation to render it inoffensive.
Many industrial processes produce odorous or toxic gases or vapors which if discharged to the atmosphere constitute either a hazard to health or a public nuisance. One solution to this problem is to collect such gases or vapors in mixture with air as they are emitted from the process and to pass the mixture through a heated reactor containing an oxidation catalyst wherein the toxic or odorous compounds are oxidized to harmless or odorless compounds such as carbon dioxide and water.
An alternative process is to pass the mixture through a burner, but direct flame incineration requires that the large volumes of gas involved are heated to high temperatures in order to combust the contaminant. Hence, considerable savings can be made in fuel costs by using catalytic combustion since catalytic reactors effect a considerable reduction in the concentration of pollutants at 150.degree.-500.degree. C. while direct flame incineration requires temperatures above this range.
However, heating large quantities of air to the temperature required for catalytic oxidation can still be costly if, as is frequently the case, the concentration of the contaminant is low and the heat released by its oxidation not contributing significantly to the overall energy requirement.
Another difficulty often encountered is that while catalytic systems can effectively remove 90%-99% of the inlet concentration of gaseous pollutant at such low temperature, in order to obtain complete removal, which, in the case of some odors particularly, is essential, often involves raising the temperature to a level at which the economic advantage of such a system no longer applies.
The object of the present invention is to provide improved and more economic processes for the destructive oxidation of air pollutants.
According to the present invention, a process for treating a pollutant-containing gaseous effluent of the kind in which the pollutant is susceptible to catalytic oxidation, to render it inoffensive, is characterised by the steps of directing a major fraction of the effluent into contact with a high surface area metal oxide capable of adsorbing substantially all of the pollutant, while simultaneously directing a minor fraction into contact with another quantity of the same metal oxide in a second treatment zone maintained at a temperature above 50.degree. C. to strip pollutant from the metal oxide in that second zone and to catalytically oxidise the pollutant, and repeatedly interchanging the role of the metal oxide from one treatment zone to the other to regenerate the adsorbency of the metal oxide containing pollutant in the second treatment zone for repeated use in the first zone. The treatment material used in both zones is a metal oxide, particularly an oxide of chromium, manganese, iron, cobalt, nickel, copper, palladium, silver, or platinum, or mixtures thereof, or oxides or mixtures thereof supported on inorganic supports such as silica gel, alumina, zeolites, capable of catalysing the oxidation of the pollutant at temperatures above 50.degree. C. and capable of adsorbing the pollutant at temperatures substantially below 100.degree. C.
The treatment material may be capable of catalysing the oxidation of pollutant at temperatures above 50.degree. C. for example at temperatures above 150.degree. C. The material may also be capable of adsorbing pollutant at temperatures above 50.degree. C. for example at a temperature between 50.degree. C. and 100.degree. C. The material may be, for example, in the form of porous granules or held together by a binder such as bentonite to form porous pellets. If a support is used it may, if conditions are appropriate, be in the form of a porous ceramic honeycomb or monolith.
In one embodiment of the invention the major fraction passes in a relatively cool state directly to and through a first treatment zone in which there is a first bed of such material capable of adsorbing pollutant from the gaseous effluent, whilst the remainder passes through a second treatment zone in which a second bed of such material with pollutant already adsorbed thereon is heated to a temperature high enough to cause desorption and partial oxidation of the adsorbed pollutant, while in a third bed, also contained within the second treatment zone complete oxidation of the adsorbed pollutant as well as the pollutant carried by the minor fraction occurs, the roles of the first and second beds of material being interchanged at intervals.
In another embodiment of the invention, the major fraction passes in a relatively cool state directly to and through a first treatment zone in which a bed of such material adsorbs pollutant therefrom, whilst the remainder, the minor fraction, passes through a second treatment zone and back to the bed. The minor fraction circulates material with pollutant adsorbed thereon from the bed through the second treatment zone, where the pollutant is catalytically oxidised, and then returns the regenerated material to the bed. The bed of material is preferably fluidised to facilitate a continuous overspill therefrom for entrainment by the minor fraction of said effluent as it passes to the second treatment zone, and eventually for continuous return to the bed.
Also according to the present invention, apparatus for treating a pollutant containing effluent of the kind in which the pollutant is susceptible to catalytic oxidation to render it inoffensive, is characterised by means defining a first treatment zone containing a high surface area metal oxide capable of catalysing the oxidation of such pollutant at temperatures above 50.degree. C. and of adsorbing pollutant at temperatures substantially below 100.degree. C., means defining a second treatment zone, conduit means for directing a major fraction of the pollutant-containing effluent through the first zone and for directing a minor fraction thereof through said second zone, means for maintaining the temperature above 50.degree. C. in the second zone and substantially below 100.degree. C. in the first zone, and means for interchanging the role of the metal oxide from one treatment zone to the other to regenerate the adsorbency of such metal oxide in the second zone for re-use of that oxide in the first zone.
One embodiment of such apparatus comprises a heater, two containers, each accommodating a bed of said material, a catalytic reactor, ducting for connecting a source of effluent to said heater and to each of the containers, means associated selectively with one or other of the containers for catalytic oxidation of pollutant adsorbed by the material supported in that container, and means adapted selectively either to cause a major fraction of effluent flowing from said source to be passed to and through one said bed for discharge therefrom and the remainder successively to and through the heater and the other bed and catalytic reactor for discharge therefrom, or to cause a major fraction of said effluent to be passed to and through the other said bed for discharge therefrom and the remainder successively to and through the heater, the one said bed and the catalytic reactor for discharge therefrom following catalytic oxidation.
Another embodiment of such apparatus comprises a container for a bed of said material in particulate form, means for facilitating the admission of effluent to said container so as to fluidize the bed, a catalytic reactor, means connecting the upper part of said bed to the reactor whereby overspill from the upper part of the bed when fluidized may be continuously directed to said reactor, means connecting said reactor to the lower part of said bed whereby overspill after passing through said reactor is continuously directed to the lower part of said bed, means for introducing a major fraction of an effluent to said container, means for causing the remainder thereof to continuously entrain said overspill and carry it at an elevated temperature to said reactor, and means for discharging effluent which emerges from said bed from said container.